The invention is concerned with wireless remote control systems. While the invention is not to be so restricted, it will be discussed in the environment of a wireless remote control system for a television receiver. Such systems normally include a transducer for converting control signals into corresponding electrical signals, decoding circuits for determining which function has been selected, and utilization circuits for performing the selected function. Various forms of noise immunity circuits are also included to enable such systems to distinguish between intentionally generated control signals and spurious signals.
Ultrasonic remote control systems have for years been a popular control medium for television receivers and are well known in the art. Several of such systems are suitable for fabrication in an integrated circuit employing digital techniques. Generally a clock is coupled to counters which sample an incoming signal to determine its frequency. Noise immunity is usually achieved by a detector and noise suppressor circuit which requires that a valid control signal have a constant amplitude for some fixed period of time. The output of the detector and noise suppressor is coupled through a pulse shaper to a clock controlled frequency counter.
Because of the high speed operation of the digital electronics which are used to sample and count frequency, the frequency sensing system may be responsive to seemingly minor variations in signal reception under certain conditions. An interruption in the incoming signal would cause a variation in the digital count for the frequency and the reoccurrence of the proper signal would be interpreted as a second actuation. This is especially significant where a selected function is noticeably altered by a detected variation of the incoming signal. Two striking examples of the effect of an interpretation of a second actuation may be seen in the off-on and mute functions. If a command to turn a set on is interrupted the result is a persistance of the off condition. This result is quite pronounced from the one expected, annoying to the viewer, and not difficult to produce. Simply moving the hand-held remote control sending unit while emitting an actuating signal may cause a relocation of an acoustic pressure node at the receiver sufficient to produce an interruption.
In an effort to prevent the possibility of such inadvertently interrupted signals, applicants have developed a novel structure for selectively reducing the responsiveness of the system. Applicants' invention concerns providing a time, after detection and identification of certain signals, during which the remote control system is rendered unresponsive to variations in the incoming signal. The off-on function is perhaps the first choice for protection by applicants' invention. Applicants' circuit embodiment provides a time to render the off-on command unresponsive to variations in the incoming signal thereby preventing unexpected interpretations of second activations to nullify the effect of the original actuating signal.